In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises Radio Base Stations (RBS) providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
A system configured for remote radio units and main units for mobile backhaul separates a radio base station into main units (MU), also referred to as base band units, and remote radio units (RRUs), connected via e.g. an optical network. The MUs may be centralized and located e.g. 10's of km from the remote radio units, which remote radio units are placed close to the radio antennas, e.g. in antenna masts. This will minimize feeder and jumper losses between antenna and remote radio units, which is often a major challenge to address in most radio communications networks, in particular to enhance the uplink capacity of mobile services. This system configured for remote radio units and main units is gaining significant interest and has some clear advantages, e.g. when it comes to installation of the remote radio units close to the antennas.
The interface between the main units and remote radio units is typically an optical Non-Return to Zero (NRZ) signal, which is a sampled In-phase Quadrature (I/Q) air interface waveform. Sampling the air waveform makes the remote radio unit implementation relatively simple but leads to very high bitrates of the optical signal, in the order of 1.25 Gbps per antenna. In parallel, advances in e.g. metro and aggregation optical networks enable seamless and common control and management planes to be established between the Packet and Optical domain, e.g. through the use of Generalized Multi-Protocol Label Switching (GMPLS). Combined with convergence of optical solutions on both sides of the metro/access network divide, e.g. Wave Division Multiplexing (WDM) or WDM-Passive Optical Network (PON), this creates new dynamics in the relations between transport and radio network solutions of tomorrow.
A Common Public Radio Interface (CPRI) specifies a Time Division Multiplexing (TDM) like protocol for Radio Base Station (RBS) configurations in a system configured for remote radio units and main units over a first layer. The application of CPRI between the main units and the remote radio units is static, i.e. determined as the RBS is deployed, and its configuration is only changed as part of a predetermined topology involving the main units and the remote radio units.
The CPRI defines a Master/Slave protocol which is used to connect a Radio Entity Controller (REC), a Radio Entity (RE). In a typical configuration, the REC will be used in a Main Unit to control RE(s) in remote radio units.
A typical location of remote radio units has a plurality of remote radio units and antennas. To reduce the required number of fibers connected to the location of remote radio units, the remote radio units are daisy-chained, i.e. wired together in sequence or in a ring, to generate a multiplexed digital signal of up to 10 Gbps, which is also the highest rate supported by the protocol generally used between a remote radio unit and a main unit, i.e. the CPRI. A more complex CPRI setup with adjacent remote radio units communicating in cascade is today used, and then a middle-remote radio unit between a main unit and remote radio unit is acting both as a Master Port, towards the remote radio unit, and a Slave Port, towards the main unit, so as to support multiplexing/demultiplexing of a CPRI stream. Whatever topology is used, the CPRI specification includes calibration facilities to enable Master-Slaves to negotiate on CPRI rates to use, including calculating offsets so that different distances between antennas and main units may be factored in before transmission over the air interface. CPRI also allows for re-calibration to occur if necessary.
The relationship between the main unit and the remote radio unit is static. Thus, there are no means for either the main unit or the remote radio unit to announce its presence and preferences to its environment, prior to connecting to another appropriate main/remote radio unit of this environment. In CPRI today, when connecting entities to one another, by definition the topology has already been determined prior to this. First, physical topologies are established between the main unit and remote radio units in a daisy chain manner. Next, the main unit and remote radio units communicate and create CPRI connections between one another according to a master and slave protocol. There are no ways to connect remote radio units and main units ad-hoc to one another.